Power-supply circuit

Abstract

A power-supply circuit includes a first transistor connected to an output terminal, a second transistor configured to output a current corresponding to an output current of the first transistor, and an amplifier configured to regulate an output voltage at the output terminal at a predetermined level. A first constant current is caused to flow through the amplifier when the output current of the second transistor is smaller than the predetermined value, and a second constant current that is greater than the first constant current is caused to flow through the amplifier when the output current of the second transistor is greater than the predetermined value.

What is claimed is: 
     
         1 . A power supply circuit, comprising:
 a first transistor connected to an output terminal;   a second transistor configured to output a current corresponding to an output current of the first transistor; and   an amplifier configured to regulate an output voltage at the output terminal at a predetermined level, wherein   a first constant current is caused to flow through the amplifier when the output current of the second transistor is smaller than the predetermined value, and   a second constant current that is greater than the first constant current is caused to flow through the amplifier when the output current of the second transistor is greater than the predetermined value.   
     
     
         2 . The power supply circuit according to  claim 1 , wherein
 the second transistor outputs a current proportional to the output current of the first transistor.   
     
     
         3 . The power supply circuit according to  claim 1 , further comprising:
 a first constant current source connected to the amplifier; and   a second constant current source connected to the amplifier through a switch, in parallel to the first constant current source, wherein   the switch is turned on when the output current of the second transistor exceeds the predetermined value.   
     
     
         4 . The power supply circuit according to  claim 3 , further comprising:
 a current comparator connected to the second transistor and a third constant current source of the predetermined value, and configured to turn on the switch when the output current of the second transistor exceeds the predetermined value.   
     
     
         5 . The power supply circuit according to  claim 1 , wherein
 the output current of the first transistor reduces fluctuation of the output voltage more quickly when the second constant current flows through the amplifier than when the first constant current flows through the amplifier.   
     
     
         6 . The power supply circuit according to  claim 1 , wherein
 the output current of the first transistor reduces an amount of fluctuation of the output voltage more when the second constant current flows through the amplifier than when the first constant current flows through the amplifier.   
     
     
         7 . The power supply circuit according to  claim 1 , wherein
 the amplifier includes third and fourth transistors connected in series, and fifth and sixth transistors connected in series and in parallel to a serial connection of the third and fourth transistors,   gates of the third and fifth transistors are connected to each other, a drain of the third transistor is connected to the gate of the third transistor, a drain of the fifth transistor is connected to gates of the first and second transistors, a gate of the fourth transistor is connected to a drain of the first transistor, and a gate of the sixth transistor is connected to a reference voltage source.   
     
     
         8 . The power supply circuit according to  claim 1 , further comprising:
 a second amplifier configured to control a gain of an output of the first amplifier, wherein   a third constant current is caused to flow through the second amplifier when the output current of the second transistor is smaller than the predetermined value, and   a fourth constant current that is greater than the third constant current is caused to flow through the second amplifier when the output current of the second transistor is greater than the predetermined value.   
     
     
         9 . The power supply circuit according to  claim 8 , further comprising:
 a third constant current source connected to the second amplifier; and   a fourth constant current source connected to the second amplifier through a second switch, in parallel to the third constant current source, wherein   the second switch is turned on when the output current of the second transistor exceeds the predetermined value.   
     
     
         10 . The power supply circuit according to  claim 8 , wherein
 the second amplifier includes a transistor having a gate connected to an output node of the amplifier and a drain connected to gates of the first and second transistors.   
     
     
         11 . A power supply circuit, comprising:
 a first transistor connected to an output terminal;   a second transistor configured to output a current corresponding to an output current of the first transistor;   an amplifier configured to regulate an output voltage of at the output terminal at a predetermined level;   a first constant current source connected to the amplifier; and   a second constant current source connected to the amplifier in parallel to the first constant current source, wherein   a connection between the second constant current source and the amplifier is established, when the output current of the second transistor exceeds a predetermined value.   
     
     
         12 . The power supply circuit according to  claim 11 , wherein
 the second transistor outputs a current proportional to the output current of the first transistor.   
     
     
         13 . The power supply circuit according to  claim 11 , further comprising:
 a switch connected between the second constant current source and the amplifier, the switch being turned on, when the output current of the second transistor exceeds the predetermined value.   
     
     
         14 . The power supply circuit according to  claim 11 , further comprising:
 a current comparator connected to the second transistor and a third constant current source of the predetermined value, and configured to turn on the switch, when the output current of the second transistor exceeds the predetermined value.   
     
     
         15 . The power supply circuit according to  claim 11 , wherein
 when the connection between the second constant current source and the amplifier is established, fluctuation of the output voltage of the first transistor decreases more quickly than when the connection is not established.   
     
     
         16 . The power supply circuit according to  claim 11 , wherein
 an amount of fluctuation of the output voltage of the first transistor is decreased when the connection between the second constant current source and the amplifier is established than when the connection is not established.   
     
     
         17 . The power supply circuit according to  claim 11 , further comprising:
 a second amplifier configured to control a gain of an output of the first amplifier;   a third constant current source connected to the second amplifier; and   a fourth constant current source connected to the second amplifier, in parallel to the third constant current source, wherein   a connection between the fourth constant current source and the second amplifier is established, when the output current of the second transistor exceeds the predetermined value.   
     
     
         18 . The power supply circuit according to  claim 17 , further comprising:
 a second switch connected between the fourth constant current source and the second amplifier, the second switch being turned on, when the output current of the second transistor exceeds the predetermined value.   
     
     
         19 . A power supply circuit, comprising:
 a first transistor connected to an output terminal;   a second transistor configured to output a current corresponding to an output current of the first transistor; and   an amplifier configured to regulate an output voltage at the output terminal at a predetermined level, wherein a current flowing through the amplifier is varied between a first current level and a second current level based on an output current of the first transistor.   
     
     
         20 . The power supply circuit according to  claim 19 , further comprising:
 a current comparator having a first input connected to the output of the second transistor and a second input connected to a reference current; and   a switch that is controlled to switch the current flowing through the amplifier to be at the first or second current level in accordance with an output of the current comparator.